1. Field of the Invention
The present invention is directed to a method and apparatus that converts image or pixel data produced by a sensor of a document scanner at a high resolution into image data at a lower resolution and, more particularly, to a system that averages pixels for conversion from 400 dots per inch to 300, 200 and 100 dots per inch.
2. Description of the Related Art
Typically, documents that are electronically scanned for facsimile transmission can be sent to the receiving apparatus at several different resolutions including at least a high resolution for graphic type documents and a low resolution for text. Documents scanned for electronic printing are typically scanned at the high resolution and printed at a medium resolution. The scanner is generally built with a sensor which will provide the highest resolution in the line direction, that is, across the document in the direction text travels, and with a variable step rate in the orthogonal direction, that is down the document. The variable step rate allows the resolution down the document to be varied as desired. If a low resolution scan is performed, the step rate is performed at the higher speed (lower resolution) and the pixel line data from the sensor, sensed at the higher resolution, must be converted into data at a comparable resolution. Typically the sensor of the scanner samples the document at a resolution of 400 dots per inch (dpi) and transmission or printing of the image typically occurs at 400 dpi, 300 dpi, 200 dpi or 100 dpi. Conventional techniques of reducing the resolution or spatial density of the image produced by the sensor include several techniques. The first, subsampling, is a technique in which pixels are dropped from the image. For example, to obtain a 100 dpi image from a 400 dpi sensor only every fourth pixel is transmitted, that is, three pixels are dropped. To transmit a 300 dpi image 3 pixels are transmitted and the fourth pixel is dropped. To transmit a 200 dpi image every other pixel is transmitted. This first technique is described in U.S. Pat. No. 4,394,693. A second technique is to use a one dimensional or a two dimensional nearest neighbor algorithm. This technique also selectively either keeps a pixel or drops a pixel depending on the location of the pixel in the output image. The third technique uses linear interpolation in one dimension or bi-linear interpolation in two dimensions. This technique also drops pixel information and only uses pixels on either side of the to be interpolated pixel for the interpolation calculations. A fourth technique is called edge decomposition and is typified by U.S. Pat. No. 4,870,497.